fix: тема доски синхронизируется между учителем и учениками в реальном времени

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/db/migrate.js

@@ -2858,6 +2858,8 @@ db.exec(`
 
 // Guest token for classroom sessions (public read-only whiteboard access)
 try { db.exec("ALTER TABLE classroom_sessions ADD COLUMN guest_token TEXT UNIQUE"); } catch {}
+// Board theme per session (synced to all participants)
+try { db.exec("ALTER TABLE classroom_sessions ADD COLUMN board_theme TEXT NOT NULL DEFAULT 'chalkboard'"); } catch {}
 
 // Persistent draw permissions (survives server restart)
 db.exec(`